Centrifugal purging and washing method for sugar mixtures and the like



Patented Apr. 18, 1944 CENTRIFUGAL PURGING AND WASHING METHOD FOR SUGARMIXTURES AND THE LIKE Eugene Roberts, Hastings, N. FL, and George E.

Stevens, Denver, 0010., assignors to The Western States Machine Company,Hamilton, Ohio, a corporation of Utah No Drawing. Application August 5,1942,

Serial No. 453,726 4 13 Claims.

This invention relates to new and useful improvements in methods forpurging and washing mixtures of crystals and syrup or mother liquor, andparticularly to the centrifugal treatment of massecuites and magmas inthe manufacture of sugar.

The principal objects .of our present invention are: To provide a newand improved method for the purging and washing of sugar massecuites andmagmas and similar mixtures. To reduce the melting of sugar crystals incentrifugal washing operations. To reduce power requirements in certaincentrifugal operations and render the same more economical thanheretofore. To increase the efiiciency of centrifugal syrup separations.And generally, to alleviate various difficulties which are nowexperienced in the centrifuging of fine grained white sugar massecuitesand the like.

Heretofore, the washing of sugar in centrifugals generally has beenconsidered as subordinate to the purging of free syrup by centrifugalforce, i. e., as a means of removing films of adherent" syrup remainingon the crystals after such purging. In accelerating a centrifugal chargeof white massecuite or the like the syrup can be seen to move readilyaway from the inside face of the sugar wall in the early stage ofacceleration, and it usually is assumed that the same action occursthroughout the sugar wall as the acceleration proceeds. Since acertainamount of wash water produces cleaner sugar and yields purer washsyrupif applied after as much as practicable of the mother liquor hasbeen purged away and while the charge is spinning nearly at its fullspeed, the universal practice heretofore has been to apply wash water inthat manner, after some 35 to 60 seconds or more of acceleration of eachcharge.

In upto-date practice on fine grain, high density, lower purity whitemassecuites, however, several conditions have been observed which seeminconsistent with premises inherent in the above. Even where the washingis done after accelerating the charge quickly to a centrifugal force ashigh as 1600 times gravity, there often is a. delay of 8 seconds or morebetween the start of washing and the time when wash syrup begins to beexpelled from the centrifugal basket. Also, wash water requirements maybe excessive; the wash syrup may not be as pure as the originalmassecuite; and to recover any wash syrup of such purity the syrupseparator (curb or ring valve type) may have to be actuated as late as10 to 15 seconds after the start of washing, with an attendant increasein green syrup purity. These conditions indicate that the wash watermust do a great deal more than to remove thin films of adherent syrupfrom the sugar grains. They are explainable by the presence, when thewashing begins, of accumulations of syrup and smear (exceedingly fine orfalse grains) in the outer zones of the sugar wall next to the basketscreens, which clog the filtration spaces and have to be loosened andwashed away before comparatively pure wash syrup can be recovered andbefore the sugar wall itself can become properly cleaned. Excessive washwater requirements are caused by this and also by the drying or cooling,or both. of syrup on sugar grains in the inner zones of the sugar wallduring the purging period.

According to the present invention, we have discovered that theforegoing objects may be achieved and that the difficulties andconditions hereinabove mentioned may be largely overcome by carrying outthe purging or elimination of the main body of syrup and smear from eachcharge, during the acceleration of the centrifugal, in conjunction withone or more regulated applications of hot aqueous liquid, such as tomobilize or dilute the syrup and smear that otherwise obstructs thefilterability and the purging and washing of the sugar wall and toassist physically the purging of the syrup and smear by centrifugalforce during the acceleration. Then, after a substantial time interval,a finish washing or cleaning operation may be performed with a clear hotliquid to finish the removal of impurities from the sugar grains whilethe charge is pervious and subject to a high centrifugal force suitablefor the type of sugar being treated.

The present method therefore is one in which charges of the massecuiteor magma are loaded and walled up in centrifugals, started onacceleration, and then washed with suitable hot wash liquid in aplurality of distinct operations or completed during the acceleration ofthe centrifugal and serves to mobilize and assist the purging of syrup,false grain or other filtration obstructions-without substantiallywashing or melting the recoverable sugar grain. In our preferredembodiments the first washing is carried out shortly after the chargehas been walled up in the centrifugal basket, before it attains half itsfull centrifugal force, so that the hot wash liquid acts upon and mixeswith the mass of syrup and smear in the charge and renders it readilypurgable as a body. The first washing and any intermediate washingrequire only a small quantity of liquid to accomplish their function,depending upon the qualities of the massecuite, and by the time thecentrifugal has reached its full speed, in the case of free-purgingmassecuites, the charge is rid of substantial obstructions of syrup andsmear, and the sugar grains also have been kept hot and moist and in areadily washable condition. A final washing from chilling the syrup ormother liquor in the charge. In the treatment of high purity massecuitesand various other mixtures, it may be preferable to carry out all of thewashing operations with a hot watery medium as disclosed in our UnitedStates Patent No. 2,223,663, such as by releasing superheated waterthrough a spray nozzle at a temperature between 100 and 150 C. suitablefor the type of massecuite being treated. In the treatment of variousmixtures, such as magmas or low grade or crystallizer massecuites, wemay prefer to use a substantially saturated syrup or sugar solution forthe first washing operation and any intermediate washing, to prevent anyincrease in green syrup or final molasses purity, and to finish thecleaning of the sugar with a hot watery medium, as aforesaid, if theprocess being carried out entails a final washing.

According-to one embodiment of this invention, a charge of sugarmassecuite is loaded into a centrifugal in the usual manner, at asuitable operation then may be performed on a pervious sugar wall havinggood filtration qualities, and it needs only to complete the cleaning ofthe sugar crystals, so that the amount of liquid required is againsmall. For the best results, this final washing is started when themaximum purg'able'amount of syrup and smear has been spun off, which,depending upon the massecuite andthe rate of acceleration, usually isnear the end of the acceleration period when the charge is'subject to ahigh'centrifugal force. At that time the wash liquidand resulting washsyrup flash quickly through the sugar wall with minimum damage to thefinest and most delicate crystals.

This method should be practiced with centrifugals which at fullspeedimpart centrifugal forces in excess of 800 times the weight of thecharge, the top speedin any case being selected according to the type ofmassecuite or magma under treatment; and the temperature of the chargesloaded into the 'centrifugals should be kept as nearly uniform aspracticable by the use of suitable massecuite heat control means. Thepresent method, however, overcomes the main problem which gave rise tothe use of rapid centrifugal acceleration on white massecuites and thelike, and makes it practical to use longer accelerations withoutsacrificing purging or washing results, at a great reduction in powerrequirements and other objectionable cost factors. In fact, although aplurality of washing operations is used for each charge, the totalquantity of wash liquid required for proper results is verysubstantially reduced, with attendant reductions of grain melt, and thetime involved for washing and drying the charge after acceleration alsois reduced. Consequently, the acceleration stage may be prolonged, toreduce power requirements and other operating costs without lengtheningthe total cycle time or reducing capacity, and at the same timesubstantial improvements in purging and washing results may be obtained.A further important advantage is that the wash syrup produced from thefinal washing operation has an extraordinarily high purity, and all ofit may be collected separately from the other centrifugal syrups toachieve a decided economy in sugar plant operations,

The wash liquid for the several washing operations may be any suitableaqueous liquid, such as water or a syrup or substantially saturatedsugar solution, but the liquid should be applied to the sugar wall at atemperature above the pan temperature of the massecuite suificient tokeep it loading speed, and when the charge has been walled up andstarted accelerating and the centrifugal curb cover has been closed (acover should be used for best results), superheated water under pressureis released through the centrifugal spray nozzle for a few seconds, say3 to 8 seconds, to complete a first washing operation before thecentrifugal attains half its full centrifugal force. After a furthersubstantial time interval during which the centrifugal continuesaccelerating and purges almost all of the syrup and smear from thecharge, say 15 to seconds, depending on the acceleration rate and thetype of massecuite, superheated water is again released through thespray nozzle until the desired degree of cleaning of the sugar wall hasbeen accomplished. When a wash syrup separation is made, the syrupseparator may be actuated at, or within a very few seconds after, thetime of starting the final washing, depending upon the massecuite, andif done correctly a wash syrup of at least the massecuite purity willresult. The drying period of the cycle follows the final washingoperation in the usual manner, but it requires less time thanheretofore.

According to another embodiment, the charge is loaded and started onacceleration, washed for a few seconds shortly after the loading,allowed to continue accelerating for several seconds without washing,then washed during another period of a few seconds, then allowed tocontinue accelerating without washing to substantially the full speed,and finally washed again for a few seconds to finish the cleaning of thesugar wall.

In still another practice, which may be preferable for low puritymassecuites, the charge is loaded, started on acceleration and thenwashed with a hot, substantially saturated syrup for one or moresuitable periods to be completed during the acceleration. After afurther interval, when the green syrup purgable by centrifugal force hasbeen spun off, a final washing is performed with a hot watery medium tofinish the cleaning of the sugar wall. The duration of this intervaldepends upon the purging qualities of the massecuite, and in the case ofcrystallizer massecuites it may be a matter of minutes instead ofseconds. 4 For efiicient and uniform operations according to thisinvention it is necessary to use automatic time controls for thecentrifugals, such as comnism to begin and terminate the several washingoperations at the desired predetermined intervals in each treatingcycle. Various well-known types of electrical or mechanical timingdevices may be used conveniently for this purpose, in various ways whichwill be apparent to those skilled in the art. Where superheated water ora mixture of steam and water is to be employed for washing a singlesupply system, such as heretofore provided for such work, may be usedfor all washing operations. Where one or more syrup washing operationsare to be performed and to be followed by a hot water or steam-waterwashing, any suitable dual wash liquid supply system may be provided andoperated under the control of the centrifugal timing mechanism. In allembodiments it is desirable to use movable covers on the centrifugalcurbs, to reduce the cooling and drying effects of air currents duringthe accelerations and high speed operations of the machines.

Eacample I The following data illustrate treatments of the same whitebeet sugar massecuite, of extra fine and somewhat irregular grain size,in part (A) with multiple water washings according to the presentinvention and in part, for comparison, (B) according to the methodregularly in use where the tests were made.

Massecuite: Brix, 91.3; purity, 92.7.

Centrifugals: 1550 R. P. M., Roberts belt-driven, with automatic timingcontrols. Acceleration period, to 1550 R. P. M., about 58".

Charge: 13 /2" deep, loaded dead=9.4 cu. ft., or 870#, containing (bycalculation) 41'I# of sugar crystals.

Wash liquid: Water at 125 0., under 55# pressure.

M (A? 1 (B) Cycle components lump 6 Regular washing m th d method 6 0Clutch in to start 1st Washing 19 8 Av. R. P. M. at start 1st washlng550 l, cog Av. duration lst washing 3: 18 Av. interval 1st to 2ndwashing Z Av. duration 2nd W&ShlI1g 10; Av. drying time 2.;

Av. total time to brake on 1'20 1'46" Average results Lbs. sugar frombasket 375 330 Lab. color grade of sugar samples... 14.2 l5. 0 Lbs.sugar per cu. ft. of massecuitc 39. 4 34 6 Quarts Wash liquid used. a11.7 l( Calculated loss of crystals ll] syrups. 42 81 These data showthat the new practice gave a 52% reduction in the loss of crystallizedsugar in syrups during centrifuging, increasing the crystal yield fromabout 79% to about 90%. Furthermore, the cycle time was shortened byabout 26", which means a great increase in the producing capacity of thecentrifugals.

Example II The following data illustrate differences in centrifugalsyrup separations when using practices such as set forth in Example I.Here a number of pans of white beet sugar massecuite were treatedsuccessively according to each of the two practices (A) and (B). Themassecuites treated therefore were not identical, but the averageresults shown nevertheless are significant in several respects:

Multiple R 5:2 washing method method No. of pans treated 1 26 Cu. ft. ofwhite massccuite. 17,767 24, (S48 Average data Massecuitc:

Refractomcter brix Apparent purity Quarts of wash liquid per cyclcm.Lbs. of same Green syrup: Apparent purity Wash syrup: Apparent purity.

1 Start of 2nd washing. 9 5 after start of washing.

According to method (A), an average of about 3 quarts of wash liquid wasused for the first washing operation. The average green syrup (motherliquor) purity, however, was not raised substantially by this washing(not more than ,/2 point in any observation), showing that the firstwashing served as an aid to purging without appreciably dissolvingsugar.

By actuating the syrup separators in method (A) at the time of startingthe second washing operation, a wash syrup was recovered having anapparent purity of 94.2-more than 1 point above the massecuite purity.In method (B), on the other hand, syrup separations made 5 seconds afterthe start of washing yielded wash syrup of only 92.4 apparent purity,which could not be recirculated to white massecuite boilings withoutlowering the massecuite purity.

The loss of crystals during centrifuging according to method (A) wasover 40% less than in method (B), and the direct recovery of sugar fromwhite massecuites was about 8% greater in (A) than in (B).

Example III Comparative tests were made, as follows, on refinery whitemassecuites, using temperaturecontrol equipment in the mixer, asuperheatedwater Washing system, and a 48" Roberts geardrivencentrifugal equipped with a ring valve syrup separator and automatictiming controls. The acceleration, from dead stop to a top speed of 1320R. P. M., occupied '70 seconds.

1. Different charges of the same massecuite (a grade B whitemassecuite)l6.'l6 cu. ft. in each chargewere purged (A) by applying thewashing medium for a period of 5 (6 /2 qts.) beginning 20" after thecentrifugal clutch was thrown in to start acceleration, and (B) withoutany washing. The syrup purged from each charge was collected and weighedas it flowed from the centrifugal, then tested, with the fol,- lowingresults:

Thus, by washing in the early stage of acceleration with about 13 lbs.of hot water, the green syrup collected in case (A) totalled 112 poundsmore than in case (B) Where there was no washing, yet the purity of thegreen syrup was not appreciably increased.

2. Different charges of 3rd inboil white massecuite were treated (A)with multiple superheated-water washings and (B) according to theregular method in use where the tests were made, and the green and washsyrups produced were collected separately, weighed and tested:

Charges: 16.76 cu. ft. (1559 pounds).

Massecuite: Purity 94.3, Ref. Brix 91.9.

Cycle components (A) (B) Clutch in to "start 1st wash" seconds.. 20 45R. P. M. at start 1st wash" 880 820 Duration lst wash seconds 17Interval 1st to 2nd wash (10.... 50 R. P. M. at start 2nd wash 1, 320Trip syrup separator Duration second wash seconds. 6 Drying time .do 1515 Results Green syrup:-

Pounds 682 502 Apparent purity 89. 8 89. 2 Wash syrup:

Pounds 104 349 Apparent purity 94. 4 93.1 Washed sugar:

Lbs. per cycle 773 724 Lbs. per cu. ft. masse 46.1 43. 2

Per cent centrii. yield on total suga n masse 57. 2 53.6

Per cent moisture 2. 7 2. 6

Color (by light transmission)... 84 80. 5 Quarts wash water 14. 3 22.'lLbs. masse. per qt. water 109 70. 6 Lbs. sugar per qt. water 54. 5 32.8

1 Start of 2nd wash. 1 7 after start of washing.

Method (A) yielded 49 pounds more of washed sugar per cycle than (B),from the same Volume of massecuite, yet the total amount of wash waterwas about less. The green syrup volume was greatly increased, withoutsubstantially increasing its purity, and the wash syrup purity was ashigh as the massecuite purity. Method (B), however, represents very goodprior practice on the type of'massecuite involved.

Some of the more important features of the methods provided by thisinvention may be summarized as follows:

Upon applying a suitable amount of superheated water or other suitablehot wash liquid to the walled-up charge as soon as the syrup leaves theface of the sugar wall, while the force of the centrifugal is less thanof its maximum force, this shot of liquid mobilizes and assist thepurging of almost all of the syrup and smear from the crystals duringcontinued acceleration, such that a sugar wall having excellentfiltration and drying qualities is obtained at the full centrifugalspeed.

The total quantity of wash liquid required for satisfactory purging andwashing results is greatly reduced, with a corresponding reduction inthe grain melt during centrifuging,

The eiiiciency of wash syrup separations is in While we have describedpreferred embodiments of'our invention in considerable detail and haveset forth data to illustrate its practical use, it is to be understoodthat the invention may be utilizedin various ways without limitation tothe details herein set forth and therefore should be ac-' corded a scopecommensurate with its contribu-' tions to the art, which are intended tobe defined by the appended claims.

We claim: l. The method of centrifuging mixtures of sugar crystals andsyrup which comprises load-. ing a charge of the mixture into asugar-cen-' trifugal and starting the acceleration thereof,

starting a washing of the charge with a, spray of hot aqueous liquidbefore the centrifugal attains half its full force and terminating saidwashing during the acceleration after applying,

a quantity of such liquid sufiicient to mobilize and assist the purgingof the syrup and any falsemixtures of sugar crystals and syrup whichcom-' prises loading a charge of the mixture into a centrifugal,accelerating the centrifugal, and washing the charge with aqueous liquidin a plurality of distinct washing operations each separated from theother by a substantial time interval, the first washing operation beingstarted shortly after the loading and continuing for only a few secondsand the last washing operation being completed after the purgable syruphas been spun from the sugar, at the full centrifugal speed.

4. The method of centrifuging and washing mixtures of sugar crystals andsyrup which comprises loading a charge of the mixture into acentrifugal, accelerating the centrifugal, applying a portion of hotwashing liquid to the charge shortly after the loading, waiting aninterval, then applying a second portion of hot washing liquid beforethe completion of acceleration, waiting another interval andthenapplying a third portion of hot washing liquid at the full centrifugalspeed, to finish the washing.

5. The method of centrifuging and washing mixtures of sugar crystals andsyrup which comprises loading a charge of the mixture intoa centrifugaland accelerating the centrifugal, applying a portion of hot washingliquid to the charge shortly after the loading to mobilize and assistthe purging of syrup and false grain, waiting an interval, and applyinganother portion of hot washing liquid to the charge to finish thewashing, at least part of said other portion being applied at the fullcentrifugal speed.

6. 'I'hemethod of centrifuging and washing mixtures of sugar crystalsand syrup which comprises loading a charge of the mixture into acentrifugal, accelerating the centrifugal, washing the charge withliquid during a limited period shortly after the loading to enhance thepurging of syrup and any false grain in the charge, and further washingthe charge with liquid during at least one later limited period at asubstantially higher centrifugal speed, the washing liquid being appliedin each instance at a temperature at least as high as the temperature ofthe charge when loaded.

7. The method of centrifuging and washing mixtures of sugar crystals andsyrup which comprises walling up a charge of the mixture in a sugarcentrifugal and starting the centrifugal acceleration, then diluting andpurging free syrup of the charge by subjecting the charge to apreliminary washin with heated aqueous liquid during an early period ofthe acceleration, and during at least one later period of thecentrifuging, at a substantially higher centrifugal speed, washing thecharge with heated water to finish the Washing.

8. The method of centrifuging and washing mixtures of sugar crystals andsyrup which comprises walling up a charge of the mixture in anaccelerating centrifugal, during acceleration diluting and purging syrupof the charge with a preliminary wash of hot substantially saturatedsugar solution, and later applying a finishing wash of heated water withthe centrifugal spinning at substantially full speed.

9. The method of centrifuging and washing mixtures of sugar crystals andsyrup which comprises walling up a charge of the mixture in anaccelerating centrifugal, for a limited period during the acceleration,beginning shortly after the loading, directing from a spray nozzle tothe charge a mist-like spray of hot water and steam, and directinganother such spray to the charge during a later limited period while thecentrifugal is spinning at substantially full speed.

10. The method of centrifuging and washing mixtures of sugar crystalsand syrup which cornprises walling up a charge of the mixture in acentrifugal and accelerating the centrifugal to a high speed sufficientto subject the charge to centrifugal force in excess of 800 times itsweight, applying a portion of hot aqueous wash liquid to the walled-upcharge before the centrifugal attains half its full force, waiting aninterval, and applying another portion of hot aqueous Wash liquid to thecharge to complete the washing when substantially the full centrifugalforce has been attained.

11. The method of centrifuging and washing mixtures of'sugar crystalsand syrup which comprises walling up a charge of the mixture in acentrifugal and accelerating the centrifugal to a high speed sufiicientto subject the charge to centrifugal force in excess of 800 times itsweight, applying a highly atomized spray of hot water and steam to thewalled-up charge for a few seconds before the centrifugal attains halfits full force, waiting an interval of several seconds, then applyinganother highly atomized spray of hot water and steam for a few seconds,waiting another interval of several seconds, and then applying a thirdhighly atomized spray of hot water and steam at the full centrifugalspeed to finish the cleaning of the sugar wall.

12. The method of centrifugally separating mixtures of sugar crystalsand syrup which comprises loading a charge of the mixture into acentrifugal and starting the acceleration of the charge in thecentrifugal, releasing superheated water through a spray nozzle to thecharge during a limited period of the acceleration beginning shortlyafter the loading, thereafter continuing the acceleration for asubstantial period, and again releasing superheated water through thenozzle to the charge during a later limited period, while the charge issubject to centrifugal force exceeding 800 times the force of gravity,to effectuate the desired final purification of the sugar.

13. The method of centrifuging each of many successive centrifugalcharges from a mass of sugar crystals and syrup which comprises wallingup and starting the acceleration of the charge in a revolving sugarcentrifugal basket, shortly thereafter releasing superheated waterthrough a spray nozzle to the accelerating charge for a. predeterminedperiod of about 3 to 8 seconds, then continuing the acceleration for apredetermined period of about 15 to seconds to finish purging the sugar,and then again releasing superheated water through the nozzle to thecharge for a predetermined period to achieve the desired final degree ofpurification of the sugar wall.

EUGENE ROBERTS. GEORGE E. STEVENS.

